This invention relates to a method of controlling an actuator by applying a driving pulse, more particularly to a method of controlling a hydraulic actuator including a solenoid valve by applying a driving pulse.
As a low cost and simple method of controlling an actuator, a method which is called "on-off control" or "bang-bang control", as shown in a 1967 publication entitled. "Control Systems Theory" by Oile I. Elgerd, published by McGraw-Hill has been conventionally known. In a case of controlling a hydraulic actuator consisting of an electromagnetic valve and a cylinder in applying the bang-bang control having an appropriate dead zone relating to the deviation between the command value and its feedback value, once a deviation between the command value and its feedback value in the control system exceeds said predetermined dead zone, the controller will immediately apply voltage to a coil of the elecromagnetic valve which will operate to reduce the deviation. Then, when the actuator is so displaced that the deviation comes within a given dead zone, the controller immediately stops application of voltage to the electromagnetic valve. However, in spite of such immediate stop of the voltage application to the electromagnetic valve, the actuator cannot stop immediately due to the inductance of the electromagnetic valve coil and the inertia of the actuator and overruns. The magnitude of this overrun depends on the operating speed of the actuator as well as said inductance and the response speed of the electromagnetic valve. Namely, the faster the operating speed of the actuator, the greater the magnitude of the overrun. When the overrun exceeds the dead zone, the controller will drive the electromagnetic valve in opposite direction to the previous operation. Consequently, the so-called limit cycle oscillation will occur unless the dead zone for the deviation was set in response to the maximum operating speed of the actuator. For this reason, with the bang-bang control, the dead zone should be enlarged in order to obtain a better responsiveness of the system by making the operating speed of the actuator faster and this means that the precision in positioning of the actuator is decreased.
As a control method having better performance in both responsiveness and precise positioning than the bang-bang control, a new servo system to operate an actuator was developed by way of converting the deviation between a command value and its feedback value into a pulse width of a digital pulse train in pulse-width-modulation, and said pulse train driving a proportional control valve in place of an on-off electromagnetic valve. The pulse train having variable duty ratios is changed through a coil of the proportional control valve into the equal analog voltage to act on the coil. This servo system enables a high speed response and a high precision, but on the other hand an expensive proportional control valve is indispensible, discouraging its use for motor vehicle control system.